【题目】If the new safety system _______ to use, the accident would never have happened.
A. had been put B. were put C. should be put D. would be put
在线课程【答案】A
【解析】考查虚拟语气。句意：如果这个新的安全系统被投入使用过的话，这个事故就不会发生了。根据主句确定是对过去情况的虚拟，所以从句用had done形式，system与put to use是动宾关系，故选A。